Conventionally, in some diesel engines, a selective reduction catalyst having a property of selectively reacting NOx (nitrogen oxides) with ammonia even in the presence of oxygen is incorporated in an exhaust pipe for flow of exhaust gas. A required amount of urea water is added upstream of the reduction catalyst to pyrolyze the urea water into ammonia and carbon dioxide according to the following formula. The ammonia is reacted as a reducing agent with NOx in the exhaust gas on the selective reduction catalyst to thereby reduce a concentration of NOx discharged.(NH2)2CO+H2O→2NH3+CO2 
For the addition of the urea water into the exhaust gas upstream of the selective reduction catalyst, a urea injector is arranged at the exhaust pipe through which the high-temperature exhaust gas flows. The urea injector is exposed to the hot environment and requires to be water-cooled so as not to be seized, so that part of the cooling water 2 cooling the engine 1 is passed to the urea injector 3 and is returned to the engine 1 as shown in FIG. 1.
The cooling water 2 from the engine 1 is branched through a three-way branching device 4 on an entry side of the urea injector 3 so that it is also adapted to be passed via a cooling-water shutoff valve 5 to a urea-water pump 6 and a urea-water tank 7, the cooling water 2 passed thereto being joined at a three-way branching device 8 with the cooling water 2 passed to the urea injector 3 and returned to the engine 1. The urea water freezing in the urea-water pump 6 and the urea-water tank 7, for example, at midwinter may be unfrozen by the cooling water 2 passed to the pump 6 and the tank 7 through opening the cooling-water shutoff valve 5. In the figure, reference numeral 9 denotes a header tank to which the cooling water 2 is poured at a highest position so as to make all of water-cooled systems including the engine 1, a radiator and the like filled with the cooling water.
A conventional art reference for this kind of cooling water circulation device is, for example, the following Patent Literature 1.